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The black box flight recorder from the doomed Germanwings plane was damaged in the impact but air investigators hope it will unlock the mystery of why the plane plunged from the sky over the Alps on Tuesday.

When investigators comb through the devastation of an aircraft crash site, the first thing they'll look for -- if there are no survivors -- is the black box.

Commercial airliners are obliged to carry two so-called black boxes, reinforced recording devices that despite the name are in fact bright orange, with reflective stripes.

One is the digital flight data recorder which gathers information about the speed, altitude and direction of the plane, while the cockpit voice recorder keeps track of conversations and other sounds in the pilots' cabin.

French Interior Minister Bernard Cazeneuve said on Wednesday that the cockpit recorder recovered from the wreckage of the Germanwings Airbus that crashed on Tuesday, killing all 150 aboard, has been found damaged and has been taken to Paris for analysis.

The recorder will hopefully have picked up the conversations between the pilots and perhaps other members of the crew throughout the flight although it's not yet known if they are of sufficient quality to be used.

Cazeneuve was however positive, saying that the box was "usable".

(Photo: BFM TV screengrab)

Investigators are set to release their initial findings later on Wednesday and they will hope the contents of the recorder will tell them why Flight 4U9525 crashed in the French Alps, without the crew having issued a distress signal.

The data black box has yet to be found on the mountain in the French Alps where the Airbus A320 went down.

Introduced in the 1960s, flight recording devices are housed in boxes built to survive extreme shocks, fire and lengthy submersion underwater.

They can survive as deep as 6,000 metres (almost 20,000 feet) underwater or an hour at 1,100 degrees centigrade. To make them easier to find, they are fitted with a beacon which can emit a signal for one month.

In January 2004, the black boxes of an Egyptian charter flight that crashed off the coast of Sharm el-Sheikh were found after a two-week search, 1,022 metres below water.

Then, after 23 months submerged at a depth of 3,900 metres in the Atlantic Ocean, the black boxes of doomed Air France flight AF447 travelling between Rio and Paris were retrieved, with the data intact, allowing investigators to determine the causes of the June 1, 2009 crash.

In France, the hunt is now on for the missing black box -- and, investigators hope, the full picture of what happened during the plane's still mysterious crash.